Plasticizers are added to a resin composition (usually a plastomer or an elastomer) to increase the flexibility, workability, or distensibility of the resin composition. The largest use of plasticizers is in the production of “plasticized” or flexible polyvinyl chloride (PVC) products. Common plasticizers include phthalates, in particular, ortho-phthalates, di-2-ethylhexyl phthalate (DEHP), diisononyl phthalate (DINP), diisodecyl phthalate (DIDP), di-octyl phthalate (DOP), dibutyl phthalate (DBP), diisobutyl phthalate (DIBP), benzylbutyl phthalate (BBP), diisoheptyl phthalate (DIHP), and many others.
Typical uses of plasticized PVC include films, sheets, tubing, coated fabrics, wire and cable insulation, jacketing, toys, flooring materials, such as, vinyl sheet flooring or vinyl floor tiles, adhesives, sealants, inks, and medical products, such as, blood bags and tubing.
Other polymer systems that use plasticizers include polyvinyl butyral, acrylic polymers, poly(vinylidene chloride), nylon, polyolefins, polyurethanes, silicon modified polymers, polysulphides, and certain fluoroplastics. Plasticizers may also be used with rubber (although often these materials fall under the definition of extenders for rubber rather than plasticizers).
Although the major commercial plasticizers for PVC are esters of phthalic acid (or the anhydride), recently, there has been an effort to find alternatives to the use of low molecular weight phthalate esters, particularly, in end uses such as food contact articles, such as, bottle cap liners and sealants, cling films, medical applications, such as, examination gloves, films, blood bags, IV delivery systems, flexible tubing, and toys.
In particular, DOP (DEHP) and shorter alkyl chain length phthalate esters have been restricted in certain applications or are the subject of increased scrutiny due to their classification as substances of very high concern (SVHC) under REACH in Europe. Their use in toys is increasingly restricted in some countries. For example, DOP (DEHP) is a versatile and widely applied plasticizer for PVC in many applications and has been for decades. DEHP together with low molecular weight phthalate plasticizers like DBP, DIBP, BBP are listed on the REACH candidate list and subject to authorization. They cannot be produced or used in Europe after the sunset date of February 2015, unless an authorization is being granted for a specific use. Thus, it must be at least contemplated that alternative plasticizers will be required. For these and most other uses of plasticized polymer systems, high molecular weight phthalates like DINP, DIDP, or DPHP have been substituted successfully for the short alkyl chain phthalate esters but there will be additional requirements to develop alternatives to DEHP in certain applications like medical or food contact. A suitable replacement would be produced from readily available materials and commercial processes and provide at least comparable performance properties.
A commercial process to produce DOP (DEHP) includes batch esterification of phthalic acid with 2-ethyl hexanol (2-EH) in the presence of an organic titanate catalyst. Esters, such as para-phthalates or terephthalates, may also be produced from other starting materials including purified terephthalic acid (PTA), terephthalic acid (TPA), or its dimethyl terephthalate (DMT) derivative. (See, for example, WO 2010/071717). However, only one monomeric ester of terephthalic acid has acquired some significance industrially as a plasticizer for PVC, namely di-2-ethylhexyl terephthalate (DEHTP or DOTP).
WO 2007/021475 is directed to the preparation of di-(2-ethylhexyl) terephthalate from terephthalic acid (TPA). More specifically, it discloses a process for the preparation of di-(2-ethylhexyl) terephthalate by the esterification of TPA with 2-ethylhexanol (2-EH) in the presence of a titanium catalyst at elevated temperature and pressure, and the removal of the water of reaction from the reaction mixture by stripping the reactor contents with an inert gas.
U.S. 2007/0161815 and WO 2008/094396 disclose the esterification of TPA with C6-C10 alcohols in a reactor equipped with a fractionation column for water removal from the refluxing 2-EH, in the presence of a titanate catalyst, operating at atmospheric pressure and a temperature of 180° C.-225° C. In an example, the reaction time was 8-9 hr to reach the targeted conversion. U.S. 2008/0183012 discloses a similar process configuration to produce di-n-butyl terephthalates by the esterification of TPA with n-butanol in the presence of an acid catalyst at atmospheric pressure, from 110° C.-220° C. temperature, and utilizing the ester product as a solvent.
WO 2008/140177 discloses the preparation method of terephthalic acid ester compositions by reacting terephthalic acid with mixtures of two different alcohols such as 2-ethylhexanol and isononyl alcohol in the presence of a titanate catalyst. In an example, the reaction is carried out at 220° C. for 9 hr at atmospheric pressure under nitrogen bubbling to reach complete conversion.
JP 2006273799 teaches that the particle size of terephthalic acid should have an average diameter of 50-300 micron when used in the esterification of terephthalic acid with 2-ethylhexanol. In an example, the required power for the mixer in the reaction vessel should be less than 5 kW/m3.
U.S. Pat. No. 7,964,658 is directed to the preparation of C4 and C5 alkyl terephthalates by the trans-esterification of dimethyl terephthalate with C4 or C5 alcohols in the presence of an organo-metallic catalyst. The alcohol feed is stepwise added to maintain at atmospheric pressure a constant reaction temperature of 185° C. and a constant reflux temperature of 65° C. In an example, the reaction is completed in 8-9 hr with 0.3 wt % dimethyl and monomethyl esters left in the product.
U.S. Pat. No. 7,361,779 mixtures of n-butyl and iso-butyl terephthalates are synthesized by the trans-esterification of dimethyl terephthalate with n-butanol and iso-butanol in the presence of a titanium compound catalyst. In an example, the reaction time was 7-11.5 hours at 110° C.-150° C. temperature, atmospheric pressure, with a reflux temperature of 65° C.-70° C., and yielded 1.4-2.0 wt. % monomethyl esters in the product.
JP 2003238479 discloses the production of terephthalates by the trans-esterification of dimethyl terephthalate with a C6-C13 monohydric alcohol in a two-step production process. In a first step, the methanol product is removed at a controlled steam temperature below the boiling point of the monohydric alcohol and by using a distillation column or a partial condenser until the methyl ester concentration becomes less than 20-30 wt %. In a second step, the reaction temperature is increased from 198° C. to 220° C. after addition of fresh monohydric alcohol to obtain a product containing less than 1 wt. % mono methyl esters.
Other background references include WO 2010/044638 and JP 04470391.
Despite these past endeavors, there exists a need for alternative plasticizers that are not subject to the same classification and restrictions as the low molecular weight phthalates and are produced from raw materials readily available worldwide in large quantities while offering the same general purpose performance properties, for example, as DEHP in existing end use applications. There further exists a need to produce these materials with improved processes for their production.